In flat sliding gate dampers, the duct or housing has an aperture in one or more of its walls into which the flat blade or gate is slidingly inserted to provide a barrier to the flow of the gaseous fluid through the housing. Sealing means are provided to avoid leakage of the gas through the closed gate and into the ambient atmosphere. Such sealing means must be effective both in the open and also in the closed positions of the damper. It is known to provide, as a sealing means, a pair of resilient curved diaphragms or seal segments positioned in back-to-back relationship, with the curved segments, in response to spring tension, pressing against the slide gate when the gate is in its inserted position, and pressing against one another when the slide gate is withdrawn. Seal segments of this type rely on the sustained resilience of the spring metal or other material of which the seal segments are made. It has been found however that after repeated usage, over extended periods of time, the seal segment material loses its resilience and as a result, when the seal gate is withdrawn, the two seal segments no longer spring back into tight leak-free engagement with one another. Moreover, if the slide gate accumulates on its surface particulate matter, as is frequently the case in precipitator usage, a very strong spring tension is required to maintain the seal segments in tight leak-free engagement with the roughened uneven surface of the slide gate. As a result there is leakage even when the slide gate is in its down or inserted position.